Original sense seems to be a measure of distance (cf. Old High German rasta, which in addition to "rest" meant "league of miles," Old Norse rost "league, distance after which one rests," Gothic rasta "mile, stage of a journey"), perhaps a word from the nomadic period. Unless the original sense is "repose," thence extended secondarily to "distance between two resting place."

The meaning "support, thing upon which something rests" is attested from 1580s. At rest "dead" is from mid-14c., on the notion of "last rest." Rest stop is from 1973. Colloquial expression to give (something) a rest "to stop talking about it" is first recorded 1927, American English.

rest

n.2

"remainder, that which is left after a separation," early 15c., from Middle French reste "remnant," from rester "to remain" (see rest (v.2)). Meaning "others, those not included in a proposition" is from 1530s.

rest

v.1

"repose, cease from action," Old English ræstan, restan "take repose by lying down; lie in death or in the grave; cease from motion, work, or performance; be without motion; be undisturbed, be free from what disquiets; stand or lie as upon a support or basis," from root of rest (n.1). Transitive senses "give repose to; lay or place, as on a support or basis" are from early 13c. Meaning "cease from, have intermission" is late 14c., also "rely on for support." Related: Rested; resting. Common Germanic, cf. Old Frisian resta, Dutch rusten, Old High German raston, German rasten, Swedish rasta, Danish raste "to rest." Resting place is from mid-14c.

rest

v.2

"to be left, remain," mid-15c., from Old French rester "to remain," from Latin restare "stand back, be left," from re- "back" (see re-) + stare "to stand" (see stet). Partially confused and merged with the other verb rest. Sense of "to continue to be" is in rest assured. Transitive sense of "to keep, cause to continue to remain" was common in 16c.-17c., "used with a predicate adjective following and qualifying the object" [Century Dictionary], hence phrase rest you merry (1540s); God rest you merry, gentlemen, often is mis-punctuated.